The present invention relates to a contact lens having improved dimensional stability, a contact lens blank having improved dimensional stability and, more particularly, relates to a spheric contact lens, a toric contact lens, a multifocal contact lens, and a contact lens blank constructed from a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate, and having either an absolute water balance ratio greater than 8 or a relative water balance ratio greater than 2 relative to a polymer of 2-hydroxyethyl methacrylate.
The prior art describes many polymers, based on acrylates or methacrylates, for use in contact lenses. For instance, U.S. Pat. No. 4,056,496 to Mancini et al. discloses a hydrogel formed by bulk polymerization of a dihydroxyalkyl acrylate or methacrylate, such as GMA; an alkyl acrylate or methacrylate; and a minor amount of an epoxidized alkyl acrylate or methacrylate. Additionally, U.S. Pat. No. 3,985,697 to Urbach teaches terpolymer hydrogels formed from hydroxyalkyl acrylate or hydroxyalkyl methacrylate, a non-water-soluble acrylate or methacrylate diester as a cross-linking agent, and an alkenoic carboxylic acid, such as acrylic or methacrylic acid. U.S. Pat. No. 3,947,401 to Stamberger discloses a bulk-polymerized water-insoluble, but water-swellable, copolymer formed from a polymerizable monoester of acrylic or methacrylic acid, such as 2-hydroxyethyl methacrylate (HEMA), and glycidyl methacrylate. Macret et al. in Polymer, 23(5), 748-753 (1982) discloses hydrogels prepared by radical polymerization of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl. methacrylate, but this document neither discloses nor suggests a contact lens having improved dimensional stability or superior water balance.
Conventional non-ionic hydrogels constructed from methyl methacrylate (MMA) copolymers derive their strength from the methacrylate polymer backbone, but depend upon the pendant hydrophilic groups of the comonomers for water content. An exemplary hydrophilic comonomer is N-vinylpyrrolidone (NVP). The structure and amount of these hydrophilic components are limited by their compatibility with the hydrophobic MMA.
HEMA-based hydrogels have a hydrophilic core that permits a water content of 38%. Higher water contents are achieved by inclusion of either methacrylic acid (MAA) comonomer in ionic hydrogels, or hydrophilic cononomers in non-ionic systems. NVP has been a key monomer in attaining water contents up to 70%, but use of this comonomer results in temperature sensitivity during manufacturing. Moreover, progressive yellowing with age and changes in optical parameters as a result of temperature-dependent dimensional changes have also been observed with lenses constructed from these compositions.
The ability of a hydrogel lens to maintain its water-saturated state is essential for maximum lens stability. All hydrogel lenses dehydrate, for water evaporates continuously from the surface of a hydrogel lens. Dehydration of a contact lens results in a change in the dimensions of the lens, hence dehydration has a direct effect upon dimensional stability. Conventional contact lenses undergo a significant degree of dehydration during use and, accordingly, have a significant degree of dimensional instability, particularly at higher water contents.
Further, rehydration is important to the dimensional stability of a contact lens. If a lens material can be constructed which absorbs water more rapidly, then the lens will more closely return to a water-saturated state during each blink, when the lens is bathed in tear fluid. Therefore, as a lens begins to dehydrate, a characteristic of rapid rehydration is extremely advantageous for maintaining saturation and maximum stability. Unfortunately, conventional contact lens development either has ignored the effect of rehydration rate upon lenses or has constructed lenses of materials with a less than optimal rate of rehydration.
As such, there remains a need for a contact lens possessing superior dimensional stability and having a low rate of dehydration coupled with a high rate of rehydration.
Accordingly, an object of the present invention is to provide a novel contact lens and contact lens blank having improved dimensional stability.
Another object of the present invention is to provide a spheric contact lens, a toric contact lens, and a multifocal contact lens, each having superior dimensional stability.
Another object of the present invention is to provide a contact lens and a contact lens blank, each with a low rate of dehydration coupled with a high rate of rehydration, relative to lenses currently available.
Still another object of the present invention is to provide a contact lens and a contact lens blank, each with a superior water balance, or ratio of dehydration to rehydration.
These objects, among others, have been accomplished by means of a contact lens and a contact lens blank constructed from a copolymer of 2,3-dihydroxypropyl methacrylate (glyceryl methacrylate hereinafter referred to as xe2x80x9cGMAxe2x80x9d) with 2-hydroxyethyl methacrylate (xe2x80x9c2-HEMAxe2x80x9d). In addition, these objects, among others, have been accomplished by means of a contact lens and a contact lens blank constructed from a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate having an absolute water balance ratio greater than 8. Still further, these objects, among others, have been accomplished by means of a. contact lens and a contact lens blank constructed from a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate that has a relative water balance ratio greater than 2 relative to a polymer of 2-hydroxyethyl methacrylate (p-HEMA).
In contrast to recent developments in rigid gas permeable contact lenses, no significant improvement has been achieved recently in hydrophilic contact lenses. All hydrophilic lenses introduced in recent years have been based on either existing materials employing new production technology or slight modifications of known compositions. This lack of progress in the soft lens field has resulted in a large variety of lens designs, but a narrow choice of lens materials and a narrow range of lens stabilities as measured by water balance ratio.
Limited choice in lens material is problematic when attempting to fit contact lenses on patients subject to a wide variety of physiological and environmental conditions. For example, an array of factors affect contact lens comfort and stability, such as, tear quantity, ambient humidity, prolonged open eye periods, and air flow around the eye. Especially difficult cases are posed by patients with dry eyes.
The dehydration of hydrophilic lenses is a major problem, affecting lens movement, lens power, oxygen permeability and comfort. Various factors including patient physiology, environment, lens design, and lens material significantly influence the rate of dehydration, as described in Andrasko, Hydration Levels and Oxygen Transmissivities of Ophthalmic Polymer In Situ, Thesis, Ohio State University, 1980, and McCarey et al. pH, Osmolarity and Temperature Effects on the Water Content of Hydrogel Contact Lenses, Contact and Intraocular Lens Medical Journal 8, 158-167, 1982. Thicker lenses also appear to dehydrate less than thinner lenses, as described in Businger et al., Die Beeirflussung der Dehydratation von hydrophilen Kontaktlinsen durch verschiedene Linsenparameter, Deutsche Optiker Zeiturig 40, 99-102 (1985).
While a variety of hydrophilic lens materials are available, they differ only slightly in their rates of dehydration, as described in Helton et al., Hydrogel Contact Lens Dehydration on Rates Determined by Thermogravimetric Analysis CLAO 17, 59-61 (1991). These factors are particularly pronounced during the cold season or in dry environments, see Andrasko et al., The Effect of Humidity on the Dehydration of Soft Contact Lenses on the Eye, Int. Cl. Clinic 7, 30 (1982) and Eng et al., The Wearing of Hydrophilic Contact Lenses Aboard a Commercial Jet Aircraft: 1Humidity Effects on Fit, Aviat. Space Environ. Med. 53, 235 (1982).
Conventional materials containing glyceryl methacrylate have been reported to have improved internal water retention over poly-HEMA, see Pescosolido, et al., Nuclear Magnetic Resonance Study of Dehydration in Glyceryl-methyl-methacrylate Contact Lens, Contactologia 15D, 64 (1993). However, this polymeric material contained MMA as a hydrophobic component, which precludes increasing the water content beyond 38%.
The ability of a hydrogel lens to maintain its saturated state is essential for lens stability. All hydrogel lenses dehydrate. Water evaporates from the surface of a hydrogel lens continuously. The amount of water loss that a lens will experience depends upon the dehydration/rehydration behavior of the particular lens material, the quantity of tears deposited on the lens with each blink, the ambient humidity, temperature and air flow around the eye.
Superior dehydration/rehydration behavior of soft lens materials provides the material with increased dimensional stability. If a soft lens material can be made to dehydrate (allow evaporation) more slowly, then the lens will remain closer to its saturated state. Equally important, but not recognized in the prior art, is the importance of rehydration. If a lens material can be made to re-absorb water more rapidly, then the lens can return to a state closer to saturation during each blink, when the lens is bathed in tear fluid. Thus, an xe2x80x9cidealxe2x80x9d soft contact lens is one constructed from a composition that is both slow to dehydrate and quick to rehydrate.
The present inventors undertook a program of research to develop a material composition for soft contact lenses having enhanced dimensional stability and superior water balance ratios (a low rate of dehydration coupled with a high rate of rehydration), both absolute and relative to p-HEMA. They discovered that copolymers of 2,3-dihydroxypropyl methacrylate with 2-hydroxyethyl methacrylate possess a low rate of dehydration coupled with a high rate of rehydration and, accordingly, would furnish contact lenses having enhanced dimensional stability.
Moreover, with the introduction of this material composition, the water content can be increased from 40% up to 70% by weight, depending on the ratio of the comonomers.